There exist various known techniques that make use of laser beams for making printed circuits, and more precisely for implementing one of the steps of the method consisting in defining conductor tracks of the printed circuit using a layer of metallization formed on an insulating backing. Such techniques can consist either in transforming the state of a layer of resist with the help of the laser beam, or else in directly ablating the layer of resist so as to reveal the layer of metallization, or finally in not using a layer of resist at all and in proceeding directly with ablation of the layer of metallization with the help of the laser beam. The present invention relates to all three of those methods which make use of a laser beam.
Naturally, in such a technique, each point or pixel on the surface of the working zone of the printed circuit panel is scanned by the laser beam which is either in an active state or in an interrupted state depending on whether the corresponding point is to be printed or not. Scanning control is associated with computer means which contain information in memory serving, for each pixel associated with its coordinates, to specify the printed or unprinted state to be achieved.
In known techniques, scanning is performed by means of a single laser beam which must therefore scan the entire surface of the panel. More precisely, the laser beam makes successive scans in one of the directions of the panel and the scans are offset in the other direction.
It will be understood that when printed circuits are of large size, the operation of scanning by means of a single laser beam in application of known techniques can require a considerable length of time. It would therefore be advantageous to provide an exposure machine in which scanning is performed simultaneously by a plurality of laser beams, each laser beam scanning a respective zone of the printed circuit.
However, it will readily be understood that using a plurality of scanning laser beams simultaneously cannot be achieved merely by juxtaposing devices each of which uses a single scanning laser beam. It will be understood that to obtain continuity in the printed circuit from one scan zone to another scan zone, specific problems are encountered.
It should also be emphasized that in order to define the shape of the conductor tracks very accurately, laser beams are used having an impact diameter on the panels of 20 .mu.m. However, because of the way energy is unevenly distributed within such a circle and also because of the way impacts overlap due to scanning, the net result is as though the surface of the panel that is printed by an impact is a square of 12.7 .mu.m.times.12.7 .mu.m. The length of 12.7 .mu.m thus constitutes the unit of definition applied to the various patterns that are made on the panel in the two orthogonal directions thereof. To conserve the precision of such definition, it is necessary for the laser beam striking the printed circuit panel to be perpendicular thereto. In addition, it is necessary in each zone to have a certain amount of latitude available for adjustment in the scan direction of the laser beam in order to be able to accommodate positioning imperfections in the various components used in generating and controlling the laser beam.